1. Field of the Invention
The present invention relates to image forming apparatus, and more particularly to an improved infinity display optical system.
2. Description of the Prior Art
U.S. Pat. No. Re. 27,356, issued May 9, 1972 for "Infinite Optical Image-Forming Apparatus" by Joseph LaRussa describes apparatus for forming, preferably at or near infinity, an image of an object or of a plurality of images optically superimposed, the apparatus having a large exit pupil and a wide angular field of view. Such apparatus is presently manufactured by Farrand Optical Co., Inc., under the trademark PANCAKE WINDOW infinity display system.
The infinity display system described in that Reissue patent employs a beam-splitting (i.e., partially transmitting), curved mirror; a first quarter-wave plate disposed on the concave side of the curved mirror; a second beam-splitting mirror disposed on the side of the first quarter-wave plate remote from the curved mirror; a second quarter-wave plate disposed on the side of the second beam-splitting mirror remote from the first quarter-wave plate; and a polarizer on the side of the second quarter-wave plate remote from the second beam-splitting mirror; the first and second quarter-wave plates having their fast axes oriented with respect to each other at a substantially integral multiple of 90.degree., and the polarizer having its plane of polarization oriented substantially at an odd integral multiple of 45.degree. to the fast axis of the second quarter-wave plate. Commonly, another polarizer is employed on the side of the curved mirror remote from the first quarter-wave plate, the direction of polarizations of said polarizer and said other polarizer being oriented with respect to each other at a second substantially integral multiple of 90.degree., both said first and second multiples of 90.degree. being even or both being odd. A detailed description of this infinity display system and its operation are given in said U.S. Pat. No. Re. 27,356, which is incorporated herein by reference.
U.S. Pat. No. 3,940,203, for "Image-Forming Apparatus" by Joseph LaRussa issued Feb. 24, 1976 also describes the preparation of a holographic analog of the above-described infinity display system, or its like, and this patent is similarly incorporated herein by reference.
As described in the aforementioned U.S. patents, an image or a plurality of images, which may or may not be linearly polarized, are projected from a display means through the nearest plane polarizer, through the beam-splitting curved mirror (usually spherical), or holographic analog thereof, and thence to the sandwich-like arrangement of quarter-wave plates, plane beam-splitter and polarizer elements toward the observer. The linearly polarized image which passes through the beam-splitting mirror is first circularly polarized in one direction by the first quarter-wave plate and then reflected by the plane beam-splitting mirror back toward the curved mirror which collimates it, passes it through to the second quarter-wave plate which reverses the circular polarization and permits the collimated image to be passed through the final plane polarizer to the observer who views the image as though it were at or near infinity. The portion of the image which is reflected by the plane beam-splitting mirror back to the collimating curved mirror and through the mirror is not seen by the observer, nor should any portion of the image pass directly through the apparatus to the observer.
Whether the optical element serving as the beam-splitting curved mirror is a true mirror or a holographic analog thereof, the components of the infinity display apparatus can be assembled to form a compact layered unit. Because in the cemented assembly of polarizers, quarter-wave plates and plane beam-splitter the bi-refringent quarter-wave plates cause the polarized light to rotate its plane of polarization, the cemented assembly is frequently termed the "bi-refringent package", and it, together with the curved mirror or its holographic analog, form the sandwich-like infinity display system.
Certain imperfections have been observed with the use of the above-noted infinity-display systems, and the following description illustrates how the present invention eliminates those imperfections. In illustrating these imperfections, the "hardest case" example has been shown, i.e. where the point of interest on the display screen is at right angles to the polaroid and bi-refringent panels.